1. Field of the Invention
The present invention relates to hydrocracking processes, and in particular to hydrocracking processes adapted to receive multiple feedstreams.
2. Description of Related Art
Hydrocracking processes are used commercially in a large number of petroleum refineries. They are used to process a variety of feeds boiling in the range of 370° C. to 520° C. in conventional hydrocracking units and boiling at 520° C. and above in the residue hydrocracking units. In general, hydrocracking processes split the molecules of the feed into smaller, i.e., lighter, molecules having higher average volatility and economic value. Additionally, hydrocracking processes typically improve the quality of the hydrocarbon feedstock by increasing the hydrogen to carbon ratio and by removing organosulfur and organonitrogen compounds. The significant economic benefit derived from hydrocracking processes has resulted in substantial development of process improvements and more active catalysts.
In addition to sulfur-containing and nitrogen-containing compounds, a typical hydrocracking feedstream, such as vacuum gas oil (VGO), contains small amount of poly nuclear aromatic (PNA) compounds, i.e., those containing less than seven fused benzene rings. As the feedstream is subjected to hydroprocessing at elevated temperature and pressure, heavy poly nuclear aromatic (HPNA) compounds, i.e., those containing seven or more fused benzene rings, tend to form and are present in high concentration in the unconverted hydrocracker bottoms.
Heavy feedstreams such as de-metalized oil (DMO) or de-asphalted oil (DAO) have much higher concentration of nitrogen, sulfur and PNA compounds than VGO feedstreams. These impurities can lower the overall efficiency of hydrocracking unit by requiring higher operating temperature, higher hydrogen partial pressure or additional reactor/catalyst volume. In addition, high concentrations of impurities can accelerate catalyst deactivation.
Three major hydrocracking process schemes include single-stage once through hydrocracking, series-flow hydrocracking with or without recycle, and two-stage recycle hydrocracking. Single-stage once through hydrocracking is the simplest of the hydrocracker configuration and typically occurs at operating conditions that are more severe than hydrotreating processes, and less severe than conventional full pressure hydrocracking processes. It uses one or more reactors for both treating steps and cracking reaction, so the catalyst must be capable of both hydrotreating and hydrocracking. This configuration is cost effective, but typically results in relatively low product yields (e.g., a maximum conversion rate of about 60%). Single stage hydrocracking is often designed to maximize mid-distillate yield over a single or dual catalyst systems. Dual catalyst systems are used in a stacked-bed configuration or in two different reactors. The effluents are passed to a fractionator column to separate the H2S, NH3, light gases (C1-C4), naphtha and diesel products boiling in the temperature range of 36-370° C. The hydrocarbons boiling above 370° C. are unconverted bottoms that, in single stage systems, are passed to other refinery operations.
Series-flow hydrocracking with or without recycle is one of the most commonly used configuration. It uses one reactor (containing both treating and cracking catalysts) or two or more reactors for both treating and cracking reaction steps. Unconverted bottoms from the fractionator column are recycled back into the first reactor for further cracking. This configuration converts heavy crude oil fractions, i.e., vacuum gas oil, into light products and has the potential to maximize the yield of naphtha, jet fuel, or diesel, depending on the recycle cut point used in the distillation section.
Two-stage recycle hydrocracking uses two reactors and unconverted bottoms from the fractionation column are recycled back into the second reactor for further cracking. Since the first reactor accomplishes both hydrotreating and hydrocracking, the feed to second reactor is virtually free of ammonia and hydrogen sulfide. This permits the use of high performance zeolite catalysts which are susceptible to poisoning by sulfur or nitrogen compounds.
A typical hydrocracking feedstock is vacuum gas oils boiling in the nominal range of 370° C. to 520° C. DMO or DAO can be blended with vacuum gas oil or used as is and processed in a hydrocracking unit. For instance, a typical hydrocracking unit processes vacuum gas oils that contain from 10V % to 25V % of DMO or DAO for optimum operation. 100% DMO or DAO can also be processed for difficult operations. However, the DMO or DAO stream contains significantly more nitrogen compounds (2,000 ppmw vs. 1,000 ppmw) and a higher micro carbon residue (MCR) content than the VGO stream (10W % vs. <1W %).
The DMO or DAO in the blended feedstock to the hydrocracking unit can have the effect of lowering the overall efficiency of the unit, i.e., by causing higher operating temperature or reactor/catalyst volume requirements for existing units or higher hydrogen partial pressure requirements or additional reactor/catalyst volume for the grass-roots units. These impurities can also reduce the quality of the desired intermediate hydrocarbon products in the hydrocracking effluent. When DMO or DAO are processed in a hydrocracker, further processing of hydrocracking reactor effluents may be required to meet the refinery fuel specifications, depending upon the refinery configuration. When the hydrocracking unit is operating in its desired mode, that is to say, producing products in good quality, its effluent can be utilized in blending and to produce gasoline, kerosene and diesel fuel to meet established fuel specifications.
In addition, formation of HPNA compounds is an undesirable side reaction that occurs in recycle hydrocrackers. The HPNA molecules form by dehydrogenation of larger hydro-aromatic molecules or cyclization of side chains onto existing HPNAs followed by dehydrogenation, which is favored as the reaction temperature increases. HPNA formation depends on many known factors including the type of feedstock, catalyst selection, process configuration, and operating conditions. Since HPNAs accumulate in the recycle system and then cause equipment fouling, HPNA formation must be controlled in the hydrocracking process.
Lamb, et al. U.S. Pat. No. 4,447,315 discloses a single-stage recycle hydrocracking process in which unconverted bottoms are contacted with an adsorbent to remove PNA compounds. Unconverted bottoms having a reduced concentration of PNA compounds are recycled to the hydrocracking reactor.
Gruia U.S. Pat. No. 4,954,242 describes a single-stage recycle hydrocracking process in which an HPNA containing heavy fraction from a vapor-liquid separator downstream of a hydrocracking reactor is contacted with an adsorbent in an adsorption zone. The reduced HPNA heavy fraction is then either recycled to the hydrotreating zone or introduced directly into the fractionation zone.
Commonly-owned U.S. Pat. No. 7,763,163 discloses adsorption of a DMO or DA0 feedstream to a hydrocracker unit to remove nitrogen-containing compounds, sulfur-containing compounds and PNA compounds. This process is effective for removal of impurities including nitrogen-containing compounds, sulfur-containing compounds and PNA compounds from the DMO or DAO feedstock to the hydrocracker unit. A separate VGO feedstock is also shown as a feed to the hydrocracker reactor along with the cleaned DMO or DAO feed. However, a relatively high concentration of HPNA compounds remains in unconverted hydrocracker bottoms.
While the above-mentioned references are suitable for their intended purposes, a need remains for improved process and apparatus for efficient and efficacious hydrocracking of heavy oil fraction feedstocks.